A week+ journey or an intensive 4-day trip with 138 km ↑ 990 m per day. Or a two-day one-way micro-tour
We have bikepacked this route route from Belgrade and back in July.
Serbian terrain is interesting in that the ground is almost completely flat to the north of Belgrade, but the farther you go to the south, the hillier it gets.
However, between Belgrade and Leskovac there is a valley that allows for riding to the south with a modest total elevation gain.
The tour has 7 intermediate stops — you could stay overnight in any of them, making use of the pre-checked accommodation, or pass by and ride more per day.
They say that real Serbian cusine can only be experienced in the very south. We tend to agree — but you should experience the gastronomy yourself.